Orthogonal frequency division multiplexing (OFDM) provides a useful way to modulate data for transmission. OFDM may be considered a form of digital multi-carrier modulation. A large number of orthogonal sub-carriers are used to carry data. Data for transmission is then divided into several parallel data streams for transmission. Each of the sub-carriers may in turn be modulated using binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), quadrature amplitude modulation (QAM), and so forth.
An OFDM system uses several carriers, or “tones,” for functions including data, pilot, guard, and nulling. Data tones are used to transfer information between the transmitter and receiver via one of the channels. Pilot tones are used to maintain the channels, and may provide information about time/frequency and channel tracking. Guard tones may be inserted between symbols to during transmission to avoid inter-symbol interference (ISI), such as might result from multi-path distortion. These guard tones also help the signal conform to a spectral mask. The nulling of the direct component (DC) may be used to simplify direct conversion receiver designs.
Selecting tone for use within a given OFDM system has proven problematic, particularly when constraints such as reuse of existing OFDM components are included.